Ok, well you saw my tips for sniper, it's better to stay alive then kill alot. More you stay alive, more kills = better kdr/acc. For sniper you wait, never take a shot even if your about to lose it, IF it's in a bad position.
For shotguns, never go out in the open obviously, this is a close ranged weapon, stick to vehicles, and houses.
For Assualt Rifles, depending on what type.. If you use a Burst Fire [My favorite because of the high ACC] Obviously dot your mouse, but make sure to drag it back down, obviously.
Best thing to do is to know the map, and know the recoil of your gun. It will take some getting used to.